Protection of AC electrical power systems of any conceivable type is comprised, and such a system does typically, but not necessarily, among others include generators of electric power, transformers, converters and networks for transmitting electric power. Furthermore, the present invention is not restricted to any particular levels of electrical powers, voltages or currents of such electrical power systems.
Sudden events in such an electrical power system, such as large jumps in load, fault occurrence or slow fault clearance, which disturb the balance of energy in the system, can cause oscillations of mechanical masses, such as accelerations of rotors in electrical machines of the system, and such oscillations disturbing said balance are referred to and here defined as power swings. In a recoverable situation these oscillations will decay and stable operation will be resumed, which may be obtained by means of different control equipment of the electrical power system, such as control systems for an electrical generator being a part of the system. However, in a non-recoverable situation, the power swings become so severe that the synchronism is lost between different parts of the system, such as generators thereof, which is a condition referred to as pole slipping or out-of-step in the literature. We will hereinafter use “pole slip” for such a condition, which accordingly is the same as “out-of-step condition”. In the case of a pole slip the excitation of electrical machines of the power system is generally intact, but there are strong oscillations of real and reactive power due to different rotational speed of involved machine rotors. Apart from the electrical phenomena, oscillations of mechanical masses also expose generators and/or other equipment of the power system to considerable pulsating mechanical stresses. Even though modern electrical power systems are designed and operate with high degree of security against power swings and even more against pole slipping, these two phenomena may occur especially during abnormal system conditions. If a pole slip is allowed to persist in one part of a power system then other electrical machines/equipment may follow and we do not only have an immediate risk of damaging bearings of rotors and other mechanical parts, but the stability of a complete electrical power system may be lost and complete blackout of the power system may be the final outcome.
It is from the above disclosure obvious that it is important to have arrangements for protecting equipment of electrical power systems against detrimental influences of power swings and pole slips, since these may otherwise cause enormous costs. In the case of severe faults it may be absolutely necessary to isolate a part of the electrical power system from the rest thereof, but it is also of importance to be able to determine whether a power swing is recoverable or not, since false tripping of such a protecting arrangement must be avoided, since that would result in a waste of considerable costs. Arrangement of this type either isolating an electrical machine, such as a generator, from the rest of the system or splitting the system at predetermined points have for this sake been proposed.
It has in the past been found that it is suitable to try to consider the electrical power system as an equivalent two machine system having at each of opposite ends thereof one theoretical electrical machine with an electro-motive force and an interconnection therebetween for being able to provide information relating to occurrence of a pole slip, i.e. that pole slip may or will occur or has occurred. The application of this model is possible if the electrical power system may be divided into two parts interconnected by a radial link-like power flow path.
The theory of this approach for obtaining protection of an electrical power system will now be briefly explained while referring to the appended FIGS. 1-3. It is shown in FIG. 1 how the electrical power system is considered as an equivalent two machine system 1 having at each opposite ends thereof one theoretical electric machine 2, 3 with an electro-motive force and an inter-connection 4 shown as an impedance. The two machines have the electro-motive forces E2 and E3 shown in FIG. 2 and here having the same magnitude. Furthermore, it is here assumed that the resistive part is quite small and thus negligible. The current I flowing between the two machines lags the vector difference E2−E3 by exactly 90° if it is assumed that the impedance is purely inductive. The angle between the two electro-motive forces E2 and E3 is δ. φ is the angle between the voltage U and the current at a given location and varies along the connection between the two electric machines 2, 3, whereas U cos φ is constant along this connection.
The angle δ between the electro-motive forces E2 and E3 of said theoretical machines is changing during a power swing, and if the power swing is non-recoverable this angle will finally go past 180°, which is defined as a pole slip. FIG. 3 is a graph showing the current I and U cos φ versus said angle δ. It appears that U cos φ will decrease when a pole slip is approaching for being zero when δ is 180°, where then the current I is the highest, which may be very harmful for equipment of the electrical power system.
Thus, it is of importance to carry out measurement of such an electrical power system so as to detect electrical parameters of interest for providing information relating to occurrence of a pole slip, and this has so far mainly been done according to two principles.
One of these principles is based on detection of rate of change of U cos φ, and this principle is further described in for instance “Innovations in the Detection of Power Swings in Electrical Networks”, Brown Boveri Review February 1981 (BBC Publication Number CH-ES 35-30.10E) by F. Ilar. Different conditions to be met for declaring a pole slip are mentioned in that document, such as the rate of change of U cos φ, which is typically in the order of 0.2-8 Hz for a pole slip condition, and passing of a threshold value of U cos φ
The other principle is based on detection of rate of change of the impedance of the interconnection of the two theoretical electric machines of the equivalent two machine system and is disclosed in for instance IEEE PSRC Tutorial 95 TP 102, “IEEE Tutorial on the Protection of Synchronous Generators”. This principle is among others based on the understanding that when a measured impedance point crosses a line in an impedance plane this will be the exact moment when a pole slip occurs. However, this theory is based upon a rather rough estimation of impedances of the electrical power systems.
It is of course an ongoing attempt to increase reliability of arrangements of the type defined in the introduction for only tripping equipment when absolutely necessary and then ensuring that this is obtained before any costly equipment of the system is harmed. It is also a desire to increase the possibilities to recover the balance of the electrical power system without disconnecting any parts thereof from the rest of the system when this is at all doable.